The invention relates in general to communication systems and more specifically to assessment of the susceptibility of a wireless communication channel to wind-induced fading.
Many wireless communication systems experience degradation in performance due to fading. Fading may be caused by one or more obstacles that block direct transmission of signals over the communications channel from the transmitter to the receiver. Fading can also occur when a signal is reflected or scattered by obstacles in the environment and two or more versions (or replicas) of the signal arrive at the receiving antenna, each along a slightly different path and each with a slightly different phase. In mobile communication systems, fading may be caused by obstacles such as buildings, trees, vehicles and other objects which alter the communication channel as the mobile unit moves in relation to both the obstacles and a fixed base station.
In fixed wireless systems, the transmitter and the receiver are arranged in locations which are fixed with respect both to each other and to scattering obstacles. Communication channels in a fixed wireless system may experience excess path loss due to the presence of stationary obstacles. The undesired effects of path loss due to stationary objects may be minimized using a variety of techniques. For example, the transmitter power may be adjusted to provide an adequate communication link. Also, the positions of the transmitter and receiver may be adjusted at the time of installation to maximize performance.
Degradation in the performance of a communication system due to changes in the communication path between the receiver and the transmitter resulting from movement of obstacles along the path is more problematic. A communication channel may be particularly susceptible to wind-induced fading if the communication path between the transmitter and the receiver includes foliage. During windy periods, movement of foliage such as on trees or bushes may result in random, time-varying changes in the propagation characteristics of the radio channel between the transmitter and receiver. Such changes often manifest themselves as random, time-varying fading of the received signal. It is therefore advantageous to position the receiver and transmitter in a manner which minimizes the amount of foliage along the path.
By visually inspecting the environment and alternative transmitter and receiver locations, and by using a test transmitter and a narrow-band measurement receiver to select a path that results in minimum path loss, it may be possible to reduce the susceptibility of a communication channel to wind-induced fading. This technique, however, does not provide a reliable method of maximizing the performance of a communication system. For example, it may be difficult to accurately assess the amount of foliage along a particular path because the observer has only limited accessibility to the location. Also, a visual inspection may be deceptive since often the combination of reflected signals cannot be predicted visually.
Another possible solution to the wind-induced fading problem includes observing a communication channel for several days. A communication channel established using a transmitter and receiver can be monitored for variations in performance and the results correlated to weather patterns. However, this potential solution may not be practical due to the required duration of the observation and the need to dedicate equipment to monitor the communication channel for a long period of time.
A partial solution is to determine the location of the scatterers which contribute to the response of the communications channel. This can be accomplished by using a wide-band channel sounder to determine the time-of-arrival of each of the direct and scattered components of the transmitted signal in combination with an apparatus capable of determining the angle-of-arrival of each of the direct and scattered components. However, this potential solution provides little, if any, information concerning the nature of the obstacles and their tendency to move in the presence of wind.
Therefore, there is need for a method, apparatus and system for predicting the performance of a communication channel based on the susceptibility of the channel to wind-induced fading. The method, apparatus and system should be easy to use and should provide an efficient way to optimize the quality of a communication channel in a wireless system by minimizing the susceptibility of the channel to wind-induced fading.